1.13.3 Gnuplot-defined variables

Gnuplot maintains a number of read-only variables that reflect the current
internal state of the program and the most recent plot. These variables begin
with the prefix "GPVAL_".
Examples include GPVAL_TERM, GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN.
Type ‘show variables all‘ to display the complete list and current values.
Values related to axes parameters (ranges, log base) are values used during the
last plot, not those currently ‘set‘.

The read-only variable GPVAL_ERRNO is set to a non-zero value if any gnuplot
command terminates early due to an error. The most recent error message is
stored in the string variable GPVAL_ERRMSG. Both GPVAL_ERRNO and GPVAL_ERRMSG
can be cleared using the command ‘reset errors‘.

Interactive terminals with ‘mouse‘ functionality maintain read-only variables
with the prefix "MOUSE_". See variables for details.

The fit mechanism uses several variables with names that begin "FIT_". It
is safest to avoid using such names. "FIT_LIMIT", however, is one that you
may wish to redefine. Under ‘set fit errorvariables‘, the error for each
fitted parameter will be stored in a variable named like the parameter, but
with "_err" appended. See the documentation on fit for details.